soc: silabs: siwx91x: Clean output names

The siwx91x need a specific firmware image format. These image end with
.rps extension. The current name of the image is zephyr.bin.rps. However,
the .bin suffix is not relevant. It makes even more sense if we consider
the output of west sign: zephyr.signed.bin.rps. We can simplify these name
by remove the .bin suffix.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>